As enc has said it depends on the software you're using, some editing programs can use the power of a CUDA enabled graphics card. HERE is a list of some of them from nVidia. There is an ATI equivalent called ATI Stream (CAL) or something like that but at this time nVidia seem to have the edge over ATI but by the time I've finished writing this over long sentence, I'm sure ATI will have caught up.

That's how changeable the nVidia v ATI scene is. There isn't a "one is better than the other". There is only this product in the same price range can be measured at half a micro second faster this week. BUT next week another product will be out to beat that one.

To really get a top notch GPU you have to pay top notch prices, that is the long and short of it. So my advice is to by an nVidia card and pay as much as you can afford. Then be satisfied with your purchase and don't look at how the one they release next week will be just that little bit better than the one you bought.